Yann asked: “So my first question is how can the buddha divide the world in 31 realms, one of which is the human plane, since mankind is only 200 000 years old.”
A fundamental idea of Buddha Dhamma (Buddhism) is that each life existing now has existed forever. At least a beginning to a “lifestream” is not discernible.
– I know that it is not compatible with science. But teachings of the Buddha have prevailed over scientific predictions so far.
– For example, even a few hundred years ago, Western science believed that the Earth was at the center of the universe. Even just over a hundred years ago, science was not aware of billions of other stars like our Sun with planetary systems like ours.
– But the Buddha had taught all that 2500 years ago.
For an introduction, see, “Dhamma and Science – Introduction.” More references are there.
Second question: “Why isn’t nibbana considered as an achievement that is somehow just above the higher of the existant realms, and how can beings in those realms fall back toward lower realms since they needed to cultivate strong positive kamma and understanding to reach that stage?”
This requires a lot more explanation.
– But one needs to understand what is discussed above first.
I would recommend the post, “Buddhism without Rebirth and Nibbāna?” after the above posts.
Buddhism (Buddha Dhamma) is VERY different from other religions. In fact, Buddha Dhamma is NOT a religion. It is a complete description of our “wider world” with 31 realms. That may sound exotic, but that is the truth.